There are many environments in which the accurate measurement of fluid flow rates is important for controlling processes and measuring device characteristics. Fluid flow rates are conventionally measured by mass flowmeters which are designed to work over a predetermined range of flow rates which are expressed in standard cubic centimeters per minute (sccm).
Of the devices which are capable of measuring low flow rates (less than 1 sccm), most presently available devices utilize one of two methods for detecting and measuring flow rate. The first method is mechanical motion of barriers placed in the flow. The second method is thermo-convection of heat sources placed in or around the flow.
The most common of the known flowmeter devices which use the first method are called "rotameters". These devices utilize a sphere of known size and density located in a tapered tube. The tube is positioned with its small diameter end downward and the fluid to be measured is introduced at the bottom of the tube. The fluid flow causes the sphere to be pushed upwards in the tube. As the fluid velocity increases, the sphere is forced higher up the tube until its weight matches the force of the flow beneath it. The flow rate can be determined by measuring the position of the sphere vertically within the tube. There are, however, several problems with presently available rotameters. One problem is that it is difficult to electrically sense the position of the sphere, and thus the devices are normally used to make visual flow rate readings by observing the position of the sphere against a scale. A second problem with the known rotameters is that before use, the devices must first be calibrated by creating a calibration chart with known flow rates. A reading taken from the rotameter must be then corrected using the calibration chart. This process introduces errors and can be time-consuming if many readings must be taken. An additional problem is that the reading of such a rotameter is very sensitive to gas density.
There are several known devices which use the second method of flowmetering--thermo-convection. The first class of these devices is the hot-wire anemometer which is a well-known device that measures the cooling of (or the current flow through) a small heated wire placed within the flow. The hot-wire anemometer is capable of providing high speed response with good accuracy for high flow rates but cannot accurately measure flow rates less than 50 sccm.
Another type of thermo-convection device is generally known as the sensor tube system. In this device, a small diameter detection channel is surrounded by three resistance heating/temperature/measuring devices such as thin film thermistors or resistive wires. One device is controlled to produce regulated heat that is directed from the device through the channel walls to the flow within. The other two sensors are placed at equidistant points upstream and downstream of the heated device. Under a no-flow condition, the amount of heat reaching each temperature sensor is equal. With a flow present, however, the fluid stream carries heat from the heated device towards the downstream sensor and thus a temperature differential develops between the upstream and downstream sensors, which temperature differential is proportional to the amount of fluid flow between the two sensors. The temperature difference can be detected as a voltage differential between the sensors by means of a bridge cirucit and an amplifier. The sensor tube arrangement is capable of measuring very low flow rates (less than 1 sccm) but it has the disadvantage that the heat to be detected must transfer by conduction through the tube walls and, accordingly, the measurement requires a long period of time to get stable readings. Thus, rapid, repetitive readings and measurements of transient flows are impossible.
Still another type of thermo-convection device utilizes thermistors placed directly wthin the gas flow rather than temperature sensors placed around it. This device is generally used in very sensitive rate of climb instruments, such as variometers used in sailplanes. Such a variometer consists of a sealed bottle with a thermistor gas flowmeter located in the bottle opening. The flowmeter measures flow into or out of the bottle caused by variations in atmospheric pressure, resulting in a rate-of-climb indication.
Variometer flowmeter instruments are capable of measuring very low flow rates (under 1 sccm) with little or no flow restriction and generally have a fast response time. However, conventional units typically use a thermistor configuration in which the thermistors are mounted on opposite sides of a printed circuit board. Thus, it is difficult to manufacture, calibrate and maintain proper calibration of the instrument throughout its life.
Therefore, it is an object of the present invention to provide a fluid flowmeter which is capable of measuring mass flow rates of less than 0.1 sccm.
It is another object of the present invention to provide a fluid flowmeter which can measure low mass flow rates with a short settling time.
It is a further object of the present invention to provide a mass flowmeter which can be quickly and easily calibrated and which maintains its calibration for long periods of time.
It is yet another object of the present invention to provide a mass flowmeter which can handle a large range of flow rates and produce accurate readings.